In the next-generation wireless communication system, demands for information rates and transmission quality have further increased. Conventionally, how to use resources in the time domain and the frequency domain has been mainly studied. In recent years, with the emergence of the multi-antenna (MIMO) technique, the new direction has been given to researchers. In the MIMO system, a transmitting side transmits signals using a plurality of antennas, while a receiving side receives the signals using a plurality of antennas. As compared with the conventional single-antenna transmission scheme, the MIMO technique is capable of remarkably increasing the channel capacity and further increasing the information transmission rate. The spatial resource can be used almost endlessly as compared with the resources in the time domain and the frequency domain, and therefore the MIMO technique can overcome issues of the conventional technique, and becomes the core technique of the next-generation wireless communication system.
FIG. 1 illustrates a configuration of a typical MIMO system. In this configuration, a transmitting side and receiving side transmit and receive signals using nT or nR antennas, respectively. On the transmitting side, first, serial/parallel conversion section 101 performs serial/parallel conversion on data queued for transmission, and the data is divided into nT data streams. Each of the nT streams corresponds to a single antenna. Before transmission, modulation/encoding sections 102-1 to 102-nT modulate and encode the data sub-streams. Then, the data sub-streams are transmitted from nT antennas 103-1 to 103-nT. On the receiving side, first, using nR antennas 104-1 to 104-nR, all the signals in space are received. Next, based on pilot signals of received signals received in antennas 104-1 to 104-nR, or using another method, channel estimation section 105 performs channel estimation, and estimates current channel characteristic matrix H. In the MIMO system, channel characteristics can be described in matrix. Then, based on channel characteristic matrix H estimated in channel estimation section 105, MIMO detection section 106 detects sub-streams by a general interference cancellation detection method, demodulates information bits transmitted from the transmitting side, and obtains original transmission data. When detecting sub-streams 1, 2, . . . , nT sequentially, MIMO detection section 106 calculates in advance equivalent SINR values (Signal to Interfering Noise Ratio) (SINR(1), SINR(1), . . . , (SINR) (nT) of the detected sub-streams.
The model of the MIMO system will be described next.
s=[s1, . . . , snt]T is assumed to be a dimensional vector of a transmission code. si is a code transmitted from an ith antenna. A signal vector of corresponding nr×1 reception antennas is y=[y1, . . . , ynr]T, and equation 1 holds.y=Hs+n  (Equation 1)
In equation 1, n=[n1, . . . , nnr]T represents zero mean white Gaussian noise of nr reception antennas. H is an nr×nt channel matrix.
To restore transmission code s from reception vectory, it is necessary to adopt MIMO reception detection and detect the signal.
Conventional detection methods include maximum likelihood detection, ZF detection, MMSE detection and BLAST detection.
In the maximum likelihood detection method, detection can be directly derived by taking statistics of noise spread of the vector. However, the complexity of the maximum likelihood detection method exponentially increases according to the number of transmission antennas, and therefore there is a problem that implementation is difficult.
A ZF detector is capable of completely canceling interference between transmission antennas, but has a problem that background noise increases at the same time.
The basic concept of an MMSE detector is to minimize a mean square error between estimated data and actual data. Considering the influence of background noise, the MMSE detector compromises between cancellation of interference between antennas and an increase in background noise, and has the performance more excellent than that of the ZF detector.
A BLAST detector (ZF-BLAST and MMSE-BLAST) is mainly configured with a linear converter and a serial interference canceller. First, data decision at the Ith antenna having the highest signal-to-noise ratio is obtained through the linear conversion. By using the data, transmission data of the Ith antenna is reconstructed, and the influence of the code is removed from the received signal. Then, data estimation of an antenna having the highest S/N ratio among remaining data is calculated, and the interference is cancelled. This operation is repeated until all data estimations are obtained.
In the conventional mobile communication, there is a problem that radio channels are uncertain and likely to change due to a poor radio channel environment. The MIMO system has the same problem. In order to reduce the code error rate and improve the system throughput, it is necessary to adopt channel coding and error correcting technique. With the channel coding, redundant information is added to original information so as to enable a receiving side to detect and correct error information. Currently, Hybrid Automatic Repeat Request (HARQ) technique is generally used as the error correcting technique. In the Hybrid Automatic Repeat Request, based on Automatic Repeat Request (ARQ) and Forward Error Correcting technique (FEC), detection and error correction is performed. The Hybrid Automatic Repeat Request technique has three types as described below. In the first type, a receiving side discards a packet that cannot be received correctly, transmits a request for retransmitting a copy of the original packet to the transmitting side using feedback information, and decodes the newly received packet independently. In the second type, a receiving side does not discard a packet with an error, combines the packet with retransmitted information, and performs decoding. This combining is also called Soft Combining. In the third type, the retransmitted information may be combined with a previously transmitted packet, but the retransmitted packet includes all necessary information upon receiving the data correctly.
When channel error correction is performed using HARQ, the transmitting side first transmits encoded information to the receiving side, and the receiving side receives the information and performs error correction. When the information can be received correctly, the receiving side receives the information, while transmitting ACK reception information to the transmitting side. When an error cannot be corrected, the receiving side transmits NACK information and a request for retransmitting the data to the transmitting side, and decodes received retransmitted data.
However, the retransmitting method in the conventional HARQ technique adapts to a single antenna, and there is a problem that it is not possible to improve transmission reliability of the system and increase the system throughput by using the HARQ technique in a multi-antenna environment.